Leeches also very much back in vogue. You can re-attach – say – a severed digit and reconnect stuff like arteries but you can’t anastomose smaller veins etc. So you get veinous insufficiency where digit swells up, goes icky and needs cutting off again. Just bung on a leech and as it feeds it stops the finger getting all oedematous while the capillaries etc regrow and re-plumb themselves in. Seeeemples! The leech saliva contains an anticoagulant which helps too.